1. Field of the Invention
The present invention relates to a thermoplastic paste intended for the production of foundry mold cores, and to a process for the production of such mold cores using the said paste.
The use of foundry mold cores of the type known as "ceramic" is well known for certain applications which demand achievement of a combination of properties and strict quality criteria such as high-temperature resistance, non-reactivity, dimensional stability and good mechanical properties. Particular examples of applications having requirements of this kind are aeronautical applications such as the foundry production of turbine blades for turbojet engines. The improvement of foundry processes, evolving from the equiaxis foundry technique to the directional solidification or the monocrystalline foundry techniques, has further added to these requirements concerning mold cores whose use and complexity have been imposed by the search for high performances for the objects which are to be produced, as is the case for example with hollow blades having internal cooling.
2. Summary of the prior art
Some known examples of compositions intended for the production of mold cores of this kind are described in Fr-A-2,37l,257, and essentially comprise fused silica, zircon flour, and cristobalite (which is a form of crystallized silica). A silicone resin is used as a binding material, and additional components such as a lubricant and a catalyst are added in small quantities. In FR-A-2,569,586 the addition of a catalyst is avoided by exploiting certain properties of the resin which is used in the production process.
Previously known compositions have not however been entirely satisfactory in certain particular applications of the directional solidification foundry technique or the monocrystalline foundry technique applied to turbine blades. Improvements are particularly sought in relation to the surface qualities and a diminution of the roughness of the mold cores obtained with the aim of making the process easier to carry out, avoiding the presence of odors due to certain products, enabling sizing of the mold cores to be carried out before firing, and also improving the production process for the mold cores, particularly by simplifying and reducing the duration of the firing cycles. In the case of certain applications, previous compositions have also resulted in problems of fragility or insufficient dimensional stability in the mold cores.
It is an object of the invention, therefore, to overcome these problems and to provide an improved composition for use in the production of foundry mold cores.
A further object is to provide an improved process for producing foundry mold cores using the said composition and involving a simplified firing cycle.